Hi all,
I need to mount a two nodes cluster with postgres as service. I have
mounted it in the past but with a shared storage and using GFS but now I
don´t have this element.

Any particular reason why you cannot use DRBD to provide that element
without a standalone SAN?

The idea is to have a master node with all the data in its own disk and
have a mechanism to replicate this data to the slave node in its own
disk. If the master goes down the slave begin to give the service and
the flow of data will go from this node to the other one. (the slave
node becomes the master one)

It is as similar a solution as PostgreSQL has available to MySQL's
replication.

It's master-slave only, though, there is no provision for master-master
replication like in MySQL (but master-master is riddled with race
conditions anyway, and you shouldn't be using it if you don't understand
the edge cases that are likely to break your app).

Have a look here for more info on PostgreSQL's replication/clustering
functionality: